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Rh occurs in limited endemic foci which tend, speaking generally, to be more numerous and larger as the Equator is approached.

Influence of latitude and season.—— In colder latitudes the association of malaria with swamps is marked; in warmer latitudes this association is much less apparent. In colder latitudes the type of disease is milder; in warmer latitudes it is apt to be more severe. In certain warm countries, as the Argentine and many of the islands of the South Pacific, malaria is entirely absent, or mild and rare. In colder latitudes it is active only during the summer or early autumn; in warmer latitudes it is perennial, certain seasons—— usually, though not invariably, the warmer, or after the rains——being the more malarial. Influence of local conditions.—— The strip of flat, waterlogged country lying along the foot of mountain ranges, the deltas of large rivers, the pool-dotted beds of dried-up streams, areas of country which have fallen out of cultivation, recently deforested lands, are, in many instances, notoriously malarial. Well-drained uplands and carefully cultivated districts, as a rule, are healthy. There are, nevertheless, instances of elevated, arid, and sandy plains which, under certain hydraulic conditions, are intensely malarial. Towns are much less malarial than villages or the open country.

Ship malaria.—— Although several instances are on record of outbreaks of what was reputed to be malaria on shipboard on the open sea, many epidemiologists refuse to accept the diagnosis as to the nature of these outbreaks, and maintain that malaria is never contracted away from the land. Mosquitoes haunt ships for some time after the latter leave port. If they are infected with the malaria parasite when they first enter the ship, or if they ingest the parasite after coming on board, they may very well communicate malaria. Endemic and epidemic fluctuations.—— From time to time malaria extends beyond its endemic foci, spreading in epidemic form over large tracts of